(1) Field of the Invention
The present invention relates to a nitride-based III-V group semiconductor epitaxial crystal for a field effect transistor.
(2) Description of the Related Art
A field effect transistor using a nitride-based III-V group semiconductor epitaxial substrate (hereinafter referred to as a GaN-FET) is a field effect transistor driven by using a GaN layer as a channel layer, and is a device which has been rapidly attracting attention in recent years for its use in high-frequency and power device applications, because this transistor has a high breakdown voltage and a higher heat-resisting property, and also each component of this transistor has a lower impact on environment, in comparison with the conventional FET having a channel made of an epitaxial semiconductor crystal such as GaAs, AlGaAs, InGaAs, InGaP, and AlInGaP, for example.
The GaN-FET is fabricated by stacking an epitaxial crystal on a base substrate in accordance with the electron beam epitaxial growth, the metalorganic vapor phase epitaxy (hereinafter referred to as the MOVPE method) and the like, and then processing the laminate into a desired device shape with the use of photolithography. Among these GaN-FETs, a transistor having an active layer made of a two-dimensional electron gas (hereinafter referred to as a 2DEG) induced at an interface between an AlGaN layer and a GaN layer is known as a GaN-HEMT, and is strongly expected to be put to practical use because of its excellent high-frequency characteristic.
As the base substrate beneath the epitaxial crystal used for such a GaN-HEMT, a substrate having a relatively small lattice mismatch with the nitride-based III-V group semiconductor crystal such as single crystal sapphire, single crystal silicon carbide (hereinafter referred to use as SiC) and single crystal silicon is used when the MOVPE method is employed for example. Among others, SiC has favorable characteristics as a substrate used for a power device with a high heating value, since SiC exhibits a good heat radiation characteristic during the operation of the device because of its large thermal conductivity in addition to its small lattice mismatch.
However, the commercially available SiC substrate is known to have a vacancy(ies) referred to as a micropipe(s) which penetrates through the substrate and has a diameter of several hundreds nanometers to several micrometers at a density of 0 to 50/cm2. Once the nitride-based III-V group semiconductor crystal has been epitaxially grown on the micropipes, they spread into the epitaxial crystal. Thus, there has existed a problem that the GaN-FET formed by using such a crystal has significantly impaired element characteristics such as lead to a short-circuit, an increase in gate leak current and degradation in pinch-off characteristic.